The present invention relates to a sheet feeding apparatus for supplying a sheet fed processing machine having a continuously running intake with successive sheets, preferably without any gaps between their leading and trailing edges. More specifically, the invention relates to such a feeder comprising retractable front sheet guides for halting the sheets and sheet advancing means adapted to operate in step with the feeding operation, said advancing means being driven from a one-turn shaft, that times the feed operation and drives the advancing means through a transmission responsible for advancing or retarding the sheets in relation to the rotation of the one-turn shaft as a function of the size of the sheets, said transmission having an output shaft moving in steps.